No one saw a thing

No one saw it happen. You stand on a crowded tube platform in London. Your two little girls jump on the train ahead of you. As you try to join them, the doors slide shut and the train moves away, leaving you behind. Everyone is lying. By the time you get to the next stop, you've convinced yourself that everything will be fine. But you soon start to panic, because there aren't two children waiting for you on the platform. There's only one. Someone...